Voluptuous adventuress Modesty Blaise is recruited by Secret Service chief Sir Gerald Tarrant to protect a shipment of diamonds to Abu Tahir, a Middle Eastern oil magnate.  The shipment is menaced, however, by Modesty's presumedly dead enemy, the archcriminial Gabriel.  Gabriel is assisted by the dotard McWhirter and the sadistic Mrs. Fothergill, to whom for chastisement he delivers abducted spy Crevier.

In Amsterdam Modesty is reunited with her former lover, Secret Service agent Paul Hagen, while her partner, Willie Garvin, is reunited with an old flame, Nicole.  Through Paul and Nicole, Modesty and Willie ascertain that Gabriel is alive.  Though Modesty and Paul are surprised in their lovenest by Gabriel's agents, Walter, Hans, and Gunther, they are rescued by Willie.  Aware that she has betrayed him, Gabriel orders Nicole's death.

En route to a rendezvous with Willie, Nicole is pursued by a tattooed man and stabbed to death.  Sought by the same assassins, Modesty disguises herself as a prostitute.  When she discovers that she does not enjoy the complete confidence of the Secret Service, Modesty and Willie determine to steal the diamonds for themselves.  Gabriel abducts Modesty.  Holding her hostage aboard his barque, he forces Willie to steal the diamonds.  Disguised as a monk, Modesty escapes with Willie and the booty.

After slaying Mrs. Fothergill, Modesty presents Abu Tahir with the shipment.  In reward for her stewardship, the sheik gives her the diamonds.  Although staked in the sands, Gabriel is rescued by McWhirter.